This invention relates to a thin-film transistor (called TFT below) and its manufacturing method. More specifically, it relates to structural technology for improving the transistor property of TFTs.
The various kinds of devices which use TFTs include an active matrix substrate for a liquid-crystal display which is formed on a transparent substrate such as one made of glass and roughly the central domain is made to be the screen display domain 81 as shown in FIG. 5 (A). In this screen display domain 81, pixels are formed with data lines 90 and scan lines 91 made of metal film such as aluminum, tantalum, molybdenum, titanium, and tungsten, silicide film, and conductive semiconductor film. On each pixel, a liquid-crystal unit 94 (liquid-crystal cell) is formed where image signals are input via a TFT 30 for image switching. For the data line 90, a data-side driving circuit 60 is constructed that is equipped with a shift register 84, a level shifter 85, a video line 87, and an analog switch 86. A scan-side driving circuit 70 equipped with a shift register 88 and a level shifter 89 are constructed for the scan line 91. On each pixel, a retention capacitor 40 is formed connected to a capacity line 92 running in parallel with the scan line 91, and this retention capacitor 40 has a function of increasing the charge retention property of the liquid-crystal unit 94. This retention capacitor 40 may be formed between a scan line 91 of the previous row and a pixel electrode.
As shown in FIG. 5(B), a CMOS circuit is constructed in the data-side and scan-side driving circuits 60 and 70, with an N-type TPT 10 and a P-type TFT 20. This kind of CMOS circuit forms an inverter circuit etc. in the driving circuits 60 and 70 with one row, two rows or more.
Therefore, in an active-matrix substrate 200, on the front side of the substrate, three kinds of TFTs are formed consisting of an N-type TFT 10 for the driving circuit, a P-type TFT 20 for the driving circuit, and an N-type TFT 30 for image switching. Here, these TFTs 10, 20, and 30 have a common basic structure and manufacturing method. Therefore, to avoid duplication of explanation, the structure and manufacturing method of the N-type TFT 10 for the driving circuit are explained with reference to FIG. 6, FIG. 7, and FIG. 8.
As shown in FIG. 6(A), in an active-matrix substrate, an insulating matrix protection film 301 is formed on the surface of a transparent substrate 30 as the base body, and a polycrystalline semiconductor film 10a (semiconductor film) is formed on the surface of this matrix protection film 301 such as polysilicon of a thickness of 50 nm for example for forming the TFT 10. On the surface of the semiconductor film 10a, a gate insulation film 13 is formed with a film thickness of 100 nm for example, and on the surface of this gate insulation film 13, a gate electrode 19 is formed. Out of the semiconductor film 10a, the domain facing the gate electrode 19 across the gate insulation film 13 is a channel domain 15 with a channel length of 5 xcexcm for example. On one side of this channel domain 15, a source domain is formed that is equipped with a low-concentration source domain 161 and a high-concentration source domain 162, and on the other side, a drain domain 17 is formed that is equipped with a low-concentration drain domain 171 and a high-concentration drain domain 172. On the front side of thus-constructed TFT 10, an interlayer insulation film 18 is formed, and a source electrode 12 formed on this interlayer insulation film 18 is electrically connected to the high-concentration source domain 162 via a contact hole 18a formed on the interlayer insulation film 18. Also, a drain electrode 14 is formed on the surface of the interlayer insulation film 18, and this drain electrode 14 is electrically connected to the high-concentration drain domain 172 via a contact hole 18b formed on the interlayer insulation film 18.
In order to manufacture thus-constructed TFT 10, as shown in FIG. 7(A), firstly a matrix protection film 301 is formed on the surface of an insulation substrate 30, and then on the entire surface of this matrix protection film, a semiconductor film 100 is formed that is made of polysilicon film of thickness 50 nm for example.
Next, a resist mask RM11 is formed on the surface of the semiconductor film 100 using the photolithography technology.
Next, a semiconductor film 1 is patterned via the resist mask RM11, and as shown in FIG. 7(B), an island-shape semiconductor film 10a (active layer) is formed.
Next, as shown in FIG. 7(C), on the surface of the semiconductor film 10a, the gate insulation film 13 is formed and is made of a silicon oxidization film with a thickness of 100 nm for example.
Next, as shown in FIG. 7(D), on the entire surface of the insulation substrate 30, a tantalum film 910 is formed which is for forming a gate electrode etc., and additionally a resist mask RM12 is formed using the photolithography technology.
Next, the tantalum film 910 is patterned via the resist mask RM12, and as shown in FIG. 7(E), a gate electrode 19 is formed of a dimension of 5 xcexcm in the channel-length direction.
Next, as shown in FIG. 7(F), low-concentration impurity ions (phosphorus ions) are implanted with a dose of 0.1xc3x971013/cm2 to 10xc3x971013/cm2 with the gate electrode 19 as a mask, forming a low-concentration source domain 161 and a low-concentration drain domain 171 self-aligned to the gate electrode. Here, the part where the impurity ions were not introduced because it was located right beneath the gate electrode remains as a semiconductor film and becomes a channel domain 15 of a channel length of 5 xcexcm.
Next, as shown in FIG. 8(A), a resist mask RM13 is formed that is wider at one side closer to the gate electrode, and high-concentration impurity ions (phosphorus ions) are implanted with a dose of 0.1xc3x971015/cm2 to 10xc3x971015/cm2, forming a high-concentration source domain 162 and drain domain 172. In this way, as shown in FIG. 8(B), a source domain 16 is formed that is equipped with the low-concentration source domain 161 and the high-concentration source domain 162, and a drain domain 17 is also formed that is equipped with the low-concentration drain domain 171 and the high-concentration drain domain 172.
Next, as shown in FIG. 8(C), an interlayer insulation film 18 is formed, and then on the interlayer insulation film 18 on the front side of the gate electrode 19, and a resist mask RM14 is formed using the photolithography technology for forming a contact hole.
Next, the interlayer insulation film 18 is etched via the resist mask RM14, and as shown in FIG. 8(D), contact holes 18a and 18b are respectively formed on the parts of the interlayer insulation film 18 corresponding to the high-concentration source domain 162 and the high-concentration drain domain 172.
Next, as shown in FIG. 8(E), an aluminum film 900 is formed on the front side of the interlayer insulation film 18 by the sputtering method etc. for constructing a source electrode etc., and additionally a resist mask RM15 is formed using the photolithography technology.
Next, the aluminum 900 is etched via a resist mask RM15, and as shown in FIG. 6(A), a source electrode 12 is formed that is made of an aluminum film electrically connected to the high-concentration source domain 162 via the contact hole 18a, and a drain electrode 14 is formed that is electrically connected to the high-concentration drain domain 172 via the contact hole 18b. 
Among such manufacturing processes, in the manufacturing process shown in FIG. 8(A), if the resist mask RM 13 is formed more widely only on the side where the drain domain 17 should be formed, as shown in FIG. 6(B), a TFT 10 can be manufactured where the low-concentration drain domain 171 exists in a domain facing the edge of the gate electrode 19 across the gate insulation film 13 on the drain domain 17 side but the high-concentration source domain 162 is formed self-aligned to the gate electrode 19 on the source domain 16 side. Because the rest of the construction in this TFT is the same with the TFT shown in FIG. 6(A), the same references are assigned to the common parts in FIG. 6(B), and their explanations are omitted.
Also, in the manufacturing process shown in FIG. 7(F), if high-concentration impurity is introduced instead of low-concentration impurity and the high-concentration source domain 162 and the high-concentration drain domain 172 are formed to be self-aligned to the gate electrode 19, a TFT 10 with a self-aligning structure can be manufactured as shown in FIG. 6(C). The rest of the construction in this TFT is also the same with the TFT shown in FIG. 6(A), the same references are assigned to the common parts in FIG. 6(C), and their explanations are omitted.
In a TFT 10 constructed in this way, a positive drain voltage relative to the source electrode 12 voltage is charged to the drain electrode 14, and a positive gate voltage is charged to the gate electrode 19. As the result, negative charge is concentrated on the interface between the channel domain 15 and the gate insulation film 13, and an N-type channel (inversion layer) is formed. At this time, when the drain voltage is small enough compared with the gate voltage, because the source domain 16 and the drain domain 17 are connected via a channel, the drain current increases as the drain voltage increases (unsaturated region) in the transistor property (current-voltage property) shown in FIG. 2(A). On the other hand, when the drain voltage becomes high enough to be very close to the gate voltage, the density of excited electrons becomes small near the drain domain 17, causing a pinch-off. Under this condition, even if the drain voltage is increased more, the drain current does not increase but becomes almost constant (a saturated region). The current value at this time is called the saturation current. Therefore, if TFT 10 is driven utilizing this saturated region, because a constant drain current can be obtained, destruction of the TFT 10 itself or surrounding circuits due to excessive current can be prevented.
However, none of the conventional TFTs shown in FIGS. 6(A), (B), and (C) has sufficient electrical properties, thus requiring further improvement.
For example, in a TFT 10 with a self-aligning structure shown in FIG. 6(C), when the drain voltage becomes high, a phenomenon (kink effect) occurs where said drain current that is supposed to be constant in the saturated region increases abnormally. The reason is as follows. First of all, when the drain voltage becomes high and the electric field between the source and the drain becomes strong, each carrier is accelerated by this electric field and comes to have large energy. Because each carrier is accelerated from the source domain 16 toward the drain domain 17, they come to have the maximum energy near the drain domain 17. A carrier with a large energy (hot carrier) collides with an atom of the semiconductor film 10a or an impurity atom and generates a pair of an electron and hole. Because the generated hole increases the voltage of the channel domain 15, the current corresponding to the injection of said hole flows from the channel domain 15 to the source domain 16. This kind of phenomenon can be understood by assuming that the channel domain 15 corresponds to the base, the source domain 16 emitter, and the drain domain 17 collector. Also, the hole current flowing from the channel domain 15 to the source domain 16 can be considered to be the base current, and the current flowing from the source domain 16 to the drain domain 17 in response to this current flowing from the channel domain 15 to the source domain 16 can be considered to be the collector current. Therefore, this phenomenon is also called bipolar action. Because of this bipolar-transistor-like behavior (bipolar action), even in the saturated region, as the drain voltage increases, the drain current rapidly increases (kink effect) as the transistor property is shown in a solid line L1 in FIG. 2(A) in conventional TFTs. As a result, the TFT itself or the surrounding circuits can be destroyed due to excessive current. Furthermore, because this kind of phenomenon becomes more significant as crystallinity of the semiconductor film 10a is increased and the drain current level of the TFT 10 is increased, conventional TFTs tend to have decreasing reliability with a higher drain current level.
On the other hand, as shown in FIG. 6(B), among the TFTs 10 that have a low-concentration drain domain 171 in the drain domain 17, in the TFT 10 where the channel length is 5 xcexcm and impurity concentration in the low-concentration drain domain 171 is 3xc3x971017 cmxe2x88x923, although the kink effect seen in a self-align-structure TFTs is improved as per the transistor property shown in a dotted broken line L2 in FIG. 2(A), it has not been sufficiently improved yet.
On the other hand, in the TFT 10 shown in FIG. 6(B), when the channel length is made 5 xcexcm and impurity concentration of the low-concentration drain domain 171 is lowered to 1xc3x971017 cmxe2x88x923, as the transistor property shown in a double-dotted broken line L3 in FIG. 2(A), the kink effect seen in self-align-structure TFTs can be sufficiently improved. However, in the TFT 10 shown in FIG. 6(B), if impurity concentration in the low-concentration drain domain 171 is lowered to 1xc3x971017 cmxe2x88x923, the drain current becomes too low, and if this TFT 10 is used in a driving circuit etc., a problem occurs that the operation speed becomes significantly lower.
An objective of this invention is to provide a TFT and its manufacturing method that prevent an abnormal increase of the drain current in the saturated region, where the drain current level in the saturated region is sufficiently high.
In order to solve said problem, in a TFT where a channel domain facing a gate electrode across a gate insulation film and a source/drain domain connected to the channel domain are formed on a semiconductor film formed on the surface of an insulating substrate, this invention may be characterized as follows. Said gate electrode and said channel domain are divided plurally in the channel-length direction. Between the divided channel domains, a low-concentration domain is formed that consists of a semiconductor film with a low impurity concentration. A low-concentration drain domain with a low impurity concentration adjoins the channel domain located closest to the drain-domain side among said divided channel domains.
According to the experiments the inventor of this application performed, the following knowledge was obtained. In a TFT where the gate electrode and the channel domain are divided plurally in the channel length direction, the low-concentration domain is formed between the divided channel domains, and a low-concentration drain domain with a low impurity concentration adjoins the channel domain located closest to the drain domain among the divided channels, even if impurity concentration is relatively high in the low-concentration domain located between the divided channel domains and the low-concentration drain domain, abnormal increase of the drain current in the saturated region can be prevented. Therefore, in a TFT according to this invention, because there is no need to lower the impurity concentration in the low-concentration domain located between the divided channel domains and the low-concentration drain domain to the extent where the drain current becomes low as for the TFT whose transistor property is shown in a double-dotted broken line L3 in FIG. 2(A), stable drain current can be obtained in the saturated region, and this drain current level is high. Therefore, a TFT that is more reliable and can achieve a high-speed operation when used in a driving circuit can be realized.
In this invention, the domain where said source domain adjoins said channel domain should be preferably a high-concentration source domain. Namely, because electric-field intensity in the drain domain should be reduced in order to prevent effectively the occurrence of the kink effect, it is preferable that only the drain-domain side is made in the LDD structure and the source-domain side is made in the self-aligning structure, thereby obtaining a higher level of drain current.
In this invention, for example, the channel-length-direction dimension of said low-concentration drain domain and the channel-length-direction dimension of said low-concentration domain located between said channel domains are made approximately equal to each other.
In this invention, among said divided gate electrodes and channel domains, the channel-length-direction dimensions of the gate electrode and channel domain located in the drain-domain side should be preferably shorter than the channel-length-direction dimension of the gate electrode and channel domain located in the source-domain side. Namely, from the viewpoint of reducing electric-field intensity in the drain-domain side, it is preferable to form the low-concentration domain located between said channel domains near the drain domain
For example, said gate electrode and channel domain are divided in two so that the ratio between the channel-length-direction dimension of the gate electrode and the channel domain located in the drain-domain side and the channel-length-dimension of the gate electrode and the channel domain located in the source-domain side becomes 1:2 to 1:10.
In this invention, impurity concentration of said low-concentration domain and said low-concentration drain domain is, for example, 3xc3x971017 cmxe2x88x923 to 3xc3x971019 cmxe2x88x923, preferably 1xc3x971018 cmxe2x88x923 to 3xc3x971019 cmxe2x88x923.
In order to manufacture a MFT of this kind of construction, after sequentially forming a semiconductor film and a gate insulation film, a gate electrode is formed that is divided plurally in the channel-length direction, on the surface of the gate insulation film. After that, by introducing low-concentration impurity to said semiconductor film using the gate electrode as a mask, plural channel domains are formed that are divided in the channel-length direction, on said semiconductor film.